The synthesis of histones during lytic SV40 infection of BSC-1 (African Green Monkey Kidney) cells has been investigated. The synthesis of all five classes of histones was stimulated to the same extent. Post-infection and increased rate of histone synthesis is detectable several hours before SV40 DNA synthesis is measurable and the rate of histone synthesis declines at a time when SV40 DNA synthesis is still at the maximum rate. Thus histone synthesis is not strictly coupled to SV40 DNA synthesis. The DNAs of two separately isolated, plaque-purified wild type strains of SV40 have been compared by restriction endonuclease mapping. One, 777, is significantly altered in the gene for the capsid protein, VP1. The difference was confirmed by analysis of tryptic peptides from the VP1s of both strains. Other differences in the genomes of the two strains were also detected. BIBLIOGRAPHIC REFERENCE: LeBlanc, D.J., and Singer, M.F.: Simian virus 40 DNA replication in Nuclear monolayers. J. Virol. 20: 78-85, 1976.